The present invention relates generally to circuit breakers and more particularly to a circuit breaker employing a pressure sensitive trip mechanism for instantaneously unlatching the circuit breaker operating mechanism in response to an overcurrent or short circuit condition.
Circuit breakers are one of a variety of overcurrent protective devices used for circuit protection and isolation. The basic function of a circuit breaker is to provide electrical system protection whenever an electrical abnormality occurs in any part of the system. In a rotary contact circuit breaker, current enters the system from a power source. The current passes through a line strap to a fixed contact fixed on the strap and then to a moveable contact. The moveable contact is fixedly attached to an arm, and the arm is mounted to a rotor that in turn is rotatably mounted in a cassette. As long as the fixed contact is in physical contact with the moveable contact, the current passes from the fixed contact to the moveable contact and out of the circuit breaker to downstream electrical devices.
In the event of an extremely high overcurrent condition (e.g. a short circuit), electromagnetic forces are generated between the fixed and moveable contacts. These electro-magnetic forces repel the movable contact away from the fixed contact. Because the moveable contact is fixedly attached to a rotating arm, the arm pivots and physically separates the fixed contact from the moveable contact.
For a given model of circuit breaker, various types of trip units may be used. For example, mounted within a circuit breaker housing, a mechanical trip unit (e.g. thermal-magnetic or magnetic) can be employed. Alternatively, an electronic trip unit can also be employed that utilizes a current transformer. In order to trip the circuit breaker, the selected trip unit must activate a circuit breaker operating mechanism. Once activated, the circuit breaker operating mechanism separates a pair of main contacts to stop the flow of current in the protected circuit. Conventional trip units act directly upon the circuit breaker operating mechanism to activate the circuit breaker operating mechanism.
In all circuit breakers, the separation of the breaker contacts due to a short circuit causes an electrical arc to form between the separating contacts. The arc causes the formation of relatively high-pressure gases as well as ionization of air molecules within the circuit breaker. Exhaust ports are conventionally employed to vent such gasses in a rotary contact circuit breaker; each phase (pole) employs two pairs of contacts, two contacts of which rotate about a common axis generally perpendicular to the current path from the line side to the load side of the circuit breaker. Each contact set in such an arrangement requires an exhaust port to expel gasses.
During an overcurrent or short circuit condition, it is desirable to trip the circuit breaker as quickly as possible in order to minimize the energy that the circuit breaker must absorb. For example, a very high level of arcing energy can develop when interrupting short circuits. Relatively severe, high level, and long lasting arcing can lead to excessive wear to the contacts as well as the arc chutes. Furthermore, if the circuit breaker can trip very quickly, higher interruption ratings can be achieved. With higher interruption ratings, overall circuit performance is improved. At the same time, any tripping system must also ensure protection for the circuit breaker and the system in the event of a single-phase condition, e.g. only one phase becomes overloaded. In a multi-phase system, a single-phase condition exists when one pole experiences a fault thereby blowing open and locking open the contacts of that pole. The remaining poles do not experience the fault and therefore their respective contacts remain closed. A single-phase condition is never desirable in a multi-phase system.
Therefore, it is desirable to provide a circuit breaker tripping mechanism that will trip a circuit breaker very quickly while ensuring protection of the circuit breaker and the electrical system should a single-phase condition occur.